Use new best practices.

- Rename escape_json_dumps to dump_js_escaped_json
- Rename escape_js_string to js_escaped_string
- Update js_escaped_string to output empty string for None
- Introduce dump_html_escaped_json
- Move dump_js_escaped_json after the pipe as new best practice
- Introduce additional uses of helpers
- Introduce new djangolib directory and move js_utils

Make escaping for json simpler and more consistent in Mako templates
- add escape_json_dumps to escape and json.dumps
- add escape_js_str to escape javascript string
- refactor Studio to use escape_json_dumps in Mako templates

TNL-2646: Escape json.dumps

course settings <nav> elements in the studio does not include accessible
label which results into much confusion for the screen reader users.

TNL-1532

TNL-1185

TNL-652

* updating vendor files
* updating class syntax (to new FA-based classes) for all UI elements
* correcting broken tests

TNL-649